1. Field Of The Invention
The invention relates to a method for monitoring the condition of a mechanical seal in an apparatus provided with a rotating part, in particular a pump, with a rotating shaft for forwarding fluid in which sound emission of the seal is continually measured in the form of analog acoustic signals at discrete times by means of a sound sensor.
2. Description of the Prior Art
Mechanical seals are, for example, used in pumps for sealing the shafts in order to avoid an escape of the fluid to be forwarded at the shaft which drives the pump wheel. Such seals are typically designed in two parts as sliding seals or as sliding ring seals, with the one part being connected in a rotationally fixed manner to the shaft, which rotates in the operating state, and the other part being secured against rotation with respect to the pump housing so that the two parts slide along one another. Normally, a sliding seal of this kind is kept in a coolant in order to dissipate the frictional heat produced. This coolant can either be the fluid to be forwarded, or a separate coolant can be provided. The coolant serves as a lubricant at the same time whereby it forms a lubricating film between the two parts of the seal sliding relative to one another, which is essentially for the functioning of the seal and its working life.
As a result of their high mechanical loading, mechanical seals of this kind represent one of the critical elements in pumps such as are, for example, used in refineries, in large chemical plants, or as feed water pumps in power plants. A failure of seals of this kind, for example, through breaking away of the lubricating film or through dry running, leads to unexpected failures of the pump and thus, often to an unexpected standstill of the entire plant. The production losses caused thereby are extremely disadvantageous from the economical point of view. However, it is also desirable for ecological and security reasons for a sudden failure of the pump as a result of wear, malfunction, or damage to a seal to be avoided as far as possible. There is therefore a need to be able to continually monitor or to assess the condition of such seals in the operating state. This is, however, made difficult due to the fact that it is often not possible to perform a visual monitoring of the seal or to install suitable sensors directly on the sliding parts of the mechanical seal with a justifiable cost and complexity.
It is known, e.g. from the German patent specification DE-C-37 07 524, to test the sliding conditions of a seal of this kind on the basis of the HF sound emission in the frequency range from 100 kHz up to at least 1 MHz. A method of this kind is based on the fact that the sound emission of the seal changes with the condition of the lubricating film. Essential for the procedure proposed in DE-C-37 07 524 is that the high frequency vibration sensor is arranged at the outer side of the machine housing in order to pick up the sound emission, i.e. is not in direct contact with the shaft seal. In accordance with the teaching of DE-C-37 07 524, the effective value, the energy value or a count value is determined from the signal derived from the acoustical measurement. This value is accumulated over a certain time interval and a frequency or amplitude distribution is derived therefrom. An assessment is then made whether the determined distribution corresponds to a normal operation of the seal.
DE-C-37 07 524 teaches the conversion of the output signal of the high frequency vibration sensor into a digital signal by means of a high speed A/D converter. In order to perform an error free digitalization at a specified maximum frequency of two MHz, the A/D converter must operate at a clock frequency of at least four MHz. High speed A/D converters which operate at such a high digitalization rate are highly specialized devices which are manufactured with high cost and complexity of the device and accordingly represent cost intensive components. The design of the apparatus for the signal processing must also be able to operate sufficiently rapidly. Taking into consideration that, as rule, a plurality of mechanical seals must be subjected to a continual and reliable monitoring in large industrial plants, the use of these high speed A/D converters and these high clock frequencies is disadvantageous from the economical point of view. They contradict the desire to achieve a cost-favorable monitoring of mechanical seals which is as economical as possible.